1. Field of the Invention
This invention relates to method of die forming by forming a material in a die, and more specifically to a method of die forming in which the material is formed to the desired shape by elastically deforming the die at the time of the forming operation (hereinafter referred to as "elastically deformable die forming").
2. Prior Art Statement
In the field of die forming it is nowadays frequently necessary to subject a material to a minute amount of forming. This is true, for example, in the production of optical products, which often have to be imparted with minute changes in shape with high precision and good reproducibility.
In carrying out such forming by conventional die forming, the die is constituted so as to resist elastic deformation and the forming operation is conducted on the premise that it will not deform. In producing a flat surface having minute irregularities (hills and valleys), therefore, it has been necessary to form the surface of the die to have a shape corresponding to the minute irregularities to be formed. With such a method, fabrication of the mold becomes increasingly difficult as the size the irregularities becomes smaller.
An example of imparting a smooth surface with minute irregularities for producing a special effect when light is reflected from the surface can be found in traditional Japanese magic mirror making. A magic mirror is made of metal having on its rear surface a pattern formed by irregularities measuring on the order of several mm. To the observer, this pattern appears to float within the normally reflected image from the front surface of the mirror.
In making such a mirror, the front surface of a cast metal plate having irregularities on the order of several mm on its rear surface is polished under strong pressure. The pressure of the polishing causes small irregularities to form on the front surface at positions corresponding to the large irregularities on the rear surface. What characterizes this technique is that the formation on the rear surface of irregularities on the order of several mm, much larger than the desired minute irregularities, makes it possible by a simple operation to form at desired locations on the front surface of the mirror the desired minute irregularities on the order of several .mu.m. The macroscopic pattern constituted of irregularities on the order of several mm can be imparted to the rear surface with ease.
Application of the magic mirror technique to various products would, however, lead to low productivity because it would be necessary to subject each product to the polishing under strong pressure. Another inherent disadvantage would be that the necessity for the irregularities on the rear surface to correspond to the minute irregularity pattern on the front surface would make it impossible to freely configure the rear surface.
The object of this invention is to provide a die forming method which effectively utilizes the elastic deformability of the die to enable the material being formed to be imparted with a desired pattern on the micron order, simply and without need for precision machining of the configuration of the die surface.